The present invention relates to disk drives and storage medium devices. In particular, the present invention relates to velocity control of a disk drive actuator during retraction in a power loss situation.
Generally, a magnetic hard disk drive (HDD) includes a magnetic read/write head and several magnetic disks, each disk having concentric data tracks for storing data. The disks are mounted on a spindle motor, which causes the disks to spin. The read/write head is typically mounted on a slider, which is mounted to a suspension or load beam. The load beam is attached to an actuator arm of an actuator, which moves the read/write head over the spinning disk during operation. As the disks spin, the slider suspended from the actuator arm “flies” a small distance above the disk surface. The slider carries a transducing head for reading from or writing to a data track on the disk.
In addition to the actuator arm, the slider suspension comprises a bearing about which the actuator arm pivots. A large scale actuator motor, such as a voice coil motor (VCM), is used to move the actuator arm (and the slider) over the surface of the disk. When actuated by the VCM, the actuator arm can be moved from an inner diameter to an outer diameter of the disk along an arc until the slider is positioned above a desired data track on the disk.
A control circuit is coupled to a coil in the VCM in order to controllably supply current to the coil. When a current is passed through the coil, a motive force is exerted on the actuator arm. The actuator arm is subjected to a force tending to accelerate the actuator arm at a rate defined by the magnitude of the current, and in a direction defined by the polarity of the current. Thus, in order to accelerate or decelerate the actuator arm until it is moving at a desired velocity and in a desired direction, it is important to know the actual direction and velocity of the actuator arm. It is known that the back electromotive force (back EMF) from the coil of the actuator is representative of the velocity and direction of movement of the actuator arm.
Parking zones in an HDD allow the read/write head to be safely landed after the hard drive has ceased operation. When an HDD is powered down, it usually performs certain operations before actually disconnecting from the external power source. One of these power down operations is to operate the actuator arm to move the head to the parking zone. If the head is not moved to the parking zone prior to power down, the head will land on the disk after the disk stops spinning, potentially damaging the disk and the read/write head.
In the event of a catastrophic shut down (i.e., external power is suddenly removed) there is no external power to perform power down procedures, including moving the head to the parking zone. Instead, power must be extracted from the spinning spindle motor. Due to the limited current available from the generator source during a power loss situation, the current used to move the actuator arm must be limited.
Various systems have been developed to control velocity during retract. One such system includes measuring the back EMF of the actuator to determine the velocity compensation required for stability. The VCM is then driven with a voltage to control the velocity of the actuator arm. A disadvantage of this system is the need for additional switched capacitor amplifiers, which makes programming the compensation over a wide range of motors difficult. Furthermore, because the VCM is driven in voltage mode, the pole of the motor limits the ramp-up of the current in the VCM, and the current cannot be directly controlled or limited.
A further approach to retracting the read/write head during power loss situations involves adjusting the voltage drive on the VCM until the velocity lies within predetermined thresholds. However, this approach is disadvantageous because the target is a range of velocities, thus allowing the velocity to be anywhere in that range. As a result, the accuracy of the final velocity of the actuator arm is inherently less than if a specific velocity is targeted.
Thus, there is a need for a system which reliably controls the retract velocity of a read/write head while limiting the current used in a power loss situation.